1. Field of the Invention
The present invention relates to a control apparatus for an induction motor having its primary winding and secondary winding connected in opposite phase, and more particularly to a control apparatus for an induction motor capable of being operated with a shunt winding characteristic.
2. Description of the Prior Art
As is well known in the art, the induction motor driven through the frequency converter composed of thyristors is often referred to as the commutatorless motor.
It is also well known that the rotational speed of such commutatorless motor depends on the output frequency of the frequency converter, as can be expressed by the following equation: ##EQU1## where N: rotational speed of the induction motor
f: output frequency of the frequency converter, and PA1 p: number of poles of the motor.
On the other hand, an upper limit is imposed on the output frequency of the frequency converter. In particular, the upper limit of the output frequency is relatively low in the case of a frequency converter in which commutations of thyristors constituting the converter are effected by utilizing an a.c. source voltage. For example, in a cyclo converter which is supplied with power from an a.c. power source of a commercial frequency to control firing angle of the thyristors for producing a sine wave voltage having a variable frequency, the upper limit of the output frequency is usually less than one third of the a.c. source frequency. The permissible highest speed of the induction motor is restricted by the above equation (1). The motor can not be driven at a high speed exceeding the permissible limit, even when load apparatus requires the high speed.
Attempts to solve the above problem have been already proposed. For example, in the article "Cyclo converter Control of the Doubly Fed Induction Motor" by WILLIS F. LONG published in "THE INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS", VOL. IGA-7, No. 1 (1971), pp. 95-100, there is disclosed an induction motor control system in which the primary and the secondary windings of the motor are connected in series in opposite phase relation relative to each other and excited by a common cyclo converter, thereby to attain a rotational frequency twice as high as the frequency of the exciting current. However, the induction motor exhibits the series winding motor characteristics which are disadvantageous in certain application.
For example, it may be contemplated that the commutatorless motor is to be used for driving a rolling mill for steel materials. In such case, the shunt motor characteristic is required for the driving motor, since the series winding motor characteristic is incompatible with load characteristic.